Improvements to DOMDiff

* Improved DOMDiff to insert a deletion marker (mw:DiffMarker meta)
  in situations where old and new subtrees are entirely different.
  This deletion marker improves selser behavior in this patch and
  other WIP patches in progress.

* The diff-marker required a fix to selser in a few places to skip
  past the mw:DiffMarker where the code wasn't already doing so.

* Overall, an additional 33 selser tests are now green (with some
  new tests failing).

Change-Id: Ia748079d2a4ab94f8577a00c66d666f727f360b8
Merge "Improvements to DOMDiff"